The SONET format (American National Standards Institute, T1.105-1988) is increasingly being accepted for the communication of many forms of digital signals. In this format, STS signals (synchronous transport signals) of various levels are defined for carrying data, at various rates, and overhead information. For example, each 125 .mu.s frame of an STS-1 signal, having a bit rate of 51.84 Mb/s, is considered as comprising 90 columns by 9 rows of 8-bit bytes, of which 3 columns are transport overhead (TOH) and the remaining 87 columns are referred to as the STS synchronous payload envelope (SPE). 84 of the 87 columns (the other 3 being used for SPE path overhead (POH) and fixed stuffing) can be divided into 7 VT (virtual tributary) groups each of 12 columns. Each VT group can accommodate various numbers and sizes of VT, for example 4 VT1.5s (i.e. four virtual tributaries each of size 1.5), 3 VT2s, 2 VT2s, or 1 VT6.
Although this invention has general applicability, for simplicity in the following detailed description reference is made only to the case of VT1.5s, it being understood that corresponding comments apply in other situations.
A VT1.5 comprises 3 columns of 9 rows of bytes, and hence 27 bytes per 125 .mu.s frame. Of these 27 bytes, in a so-called floating VT mode one byte (the first byte, V1, V2, V3, or V4 in successive frames of a 4-frame superframe) serves as a VT payload pointer, and the remaining 26 bytes (the VT SPE) can accommodate the 193 bits per 125 .mu.s frame of a conventional DS-1 signal, together with signalling information, VT POH, and fixed stuffing. In a convenient form of mapping, referred to as byte sync, the 24 bytes or DS-0 channels (192 bits) in the DS-1 frame are mapped directly into 24 bytes of the VT SPE, the remaining 2 bytes being used for the signalling, the DS-1 framing bit, the VT POH information byte V5, and stuffing.
A problem arises with this mapping from the facts that these 2 remaining bytes are adjacent one another in the VT SPE, the DS-1 signal is asynchronous to the VT, and a phase comparison is made between the VT SPE and the asynchronous DS-1 signal at a fixed point in the VT SPE mapping. Because the DS-1 signal is asynchronous, stuffing must be performed to compensate for its frequency difference relative to the synchronous network (VT frequency justification). This results in the position of these 2 remaining bytes relative to the VT SPE changing progressively over time. In known manner, the asynchronous DS-1 data is written into a data buffer from which it is subsequently read synchronously, to accommodate the stuffing.
Thus for example the phase comparison is made, i.e. the fill of the buffer is evaluated, at the time of the VT payload pointer byte V1. If for example the byte V5 immediately follows the byte V1 and the data buffer is becoming full so that a negative stuff and corresponding pointer adjustment is required, then a negative stuff is performed at the next stuffing opportunity. Consequently a stuff byte R, rather than a DS-1 data byte from the data buffer, is stuffed into the V3 byte position. Accordingly, at the time of the next byte V1 the data buffer fill evaluation still indicates that a negative stuff is required. Again this is performed at the next opportunity, but the data buffer is only emptied by one bit, namely the DS-1 framing bit, because the next byte to be stuffed in the V3 byte position is the signalling byte (the second of the 2 remaining bytes referred to above) rather than a data byte. Accordingly, the next evaluation of the data buffer fill still indicates that a negative stuff is required. This third negative stuff is performed at the next opportunity whereupon a DS-1 data byte is read out from the data buffer and its fill level, as evaluated at the time of the byte V1, is reduced.
At other times in the 125 .mu.s frames, only a single negative stuff takes place because a data byte from the data buffer is immediately stuffed into the V3 byte position.
Thus there is an irregularity in the occurrence of stuffing to accommodate the asynchronous DS-1 data in the VT, in that most of the time single stuffs are performed, and occasionally three successive stuffs are performed in the above circumstances. This irregularity constitutes a jitter in the decoded (desynchronized) DS-1 signal, which is undesirable. Although this irregularity is explained above in relation to negative stuffing, it also occurs in the converse situation with positive stuffing.
Although this problem could be reduced by providing an additional stage of buffering for the VT SPE data, this is undesirable because it would introduce an additional data propagation delay. Furthermore, it is very desirable to integrate all of the circuitry needed for conversion between the DS-1 and VT1.5 signal formats in a single integrated circuit of as small a size as possible. The additional integrated circuit chip area which would be required for such an additional stage of buffering presents a disadvantage in this respect. Accordingly, the provision of an additional stage of buffering is not a particularly practical solution to the problem.
An object of this invention, therefore, is to provide an improved method of synchronizing data in which this problem is reduced or substantially avoided.